1. Field of the Invention
The present invention relates to a polycarbonate and a process for producing the same, particularly to a polycarbonate being excellent in color tone or transparency as well as heat resistance and water resistance, and a process for producing the same.
2. Description of the Related Arts
Generally, as processes for producing polycarbonate (hereinafter referred to as PC), a method of directly reacting an aromatic dihydroxy compound such as bisphenol A with phosgene (the Interfacial Method), and a method of transesterifying an aromatic dihydroxy compound such as bisphenol A and a carbonic ester such as diphenylcarbonate, in molten a state, (the Melting Method) are known.
However, the Interfacial Method has problems in (i) that toxic phosgene must be used, (ii) that the equipment for production is corroded with chlorine-containing compounds including hydrogen chloride and sodium chloride which are by-produced, and (iii) that impurities such as sodium hydroxide contaminating into the resin which adversely affect the physical properties of polymers is difficult to be separated away.
Though the Melting Method, on the other hand, has an advantage in being capable of producing a PC at a lower cost than in the Interfacial Method, it has a substantial disadvantage in not being free from coloring of the resin because the reaction usually continues for a long time at so high a temperature as 280.degree. to 310.degree. C.
In the Melting Method, various improvements have been proposed to reduce the coloring. For example, Japanese Patent Publication No. 39972/1986 and Japanese Patent Application Laid-Open No. 223036 disclose methods of using specified catalyst. Japanese Patent Application Laid-Open Nos. 151236/1986 and 158719/1987 and the like disclose methods of adding antioxidants at the latter stage of the reaction. Moreover, Japanese Patent Application Laid-Open No. 62522/1986 and the like disclose the use of twin-screw vented kneading extruder at the latter stage of the reaction, and Japanese Patent Application Laid-Open No. 153925/1990 and the like disclose the use of a horizontal stirring polymerization reactor, which are improvements in process. Further, Japanese Patent Application Laid-Open No. 175722/1990 discloses a method of controlling the hydrolizable chlorine content in the monomer to be under a prescribed level. Actually, however, the problem in coloring has not been overcome completely, and a satisfactory PC has not been obtained yet.
Furthermore, methods of limiting the material of reactor to a specific metal have been disclosed. Conventionally, when a reactor made of stainless steel is used, the resulting PC has been liable to be colored into yellow or yellowish brown. In order to improve that, U.S. Pat. No. 4,383,092 discloses a method of using tantalum, nickel or chromium as the material. Besides, Japanese Patent Application Laid-Open No. 7328/1992 discloses a method of buff-polishing stainless steel reactors, and Japanese Patent Laid-Open No. 7329/1992 discloses a method of acid washing stainless steel reactors.
However, these methods could not overcome the elusion of metals, and a satisfactory PC having an excellent color tone or transparency has not been obtained yet. It is indeed possible to prevent the elusion of metals by using a reactor in which all the reaction system is lined with glass. However, when the process is shutdown and the polymer is cooled to solidness, the glass breaks due to the difference in expansion coefficient. In this connection, this system is substantially not sufficient in mechanical strength, and cannot be employed for industrial use.
In view of the above circumstances, the present inventors have repeated intensive studies to develop a PC being excellent in color tone and transparency as well as heat resistance and water resistance, and a process for efficiently producing the PC.
As the result, it was found that the above object can be attained by controlling the amount of specific residual metals to be under a certain level, in the process for producing PC by transesterification. The present invention has been accomplished on the basis of this knowledge.